Embodiments relate to production systems and methods to produce olefin copolymers such as copolymers in which a low crystallinity polymer is formed in the presence of a semi-crystalline polymer.
Impact copolymers (“ICPs”) are generally produced in a series of polymerization reactors, such as, for example, as disclosed in U.S. Pat. No. 7,851,554, which is hereby incorporated herein by reference. Briefly, a polypropylene reactor system, such as, for example, a slurry bulk propylene loop reactor or reactors, forms a homopolymer (“hPP”) matrix, followed by a second reactor system in series, such as, for example, a gas phase fluidized bed reactor(s) (“GPR”), where an ethylene-propylene biopolymer (e.g., an ethylene propylene rubber (“EPR”)), is formed in the presence of the matrix, for example, within the pores of the matrix.
The close-coupled continuous nature of this process often imposes limitations on the quality of the EPR that can be formed in the GPR; for example, the maximum molecular weight (“MW”) of the EPR is often limited by the amount of chain terminating agent, e.g., hydrogen, that is carried over from the slurry bulk propylene loop reactor(s) into the GPR. The ratio of MW between the EPR and the hPP matrix as determined by intrinsic viscosity measurements (“IV ratio” or “IVR”), is often used to characterize the ICP product and has significant impact on the product processing and final application properties.
Special equipment and additional energy are required between the slurry bulk propylene loop reactor(s) and GPR to perform a low pressure separation to remove this excess hydrogen if it is desired to produce high MW EPR. As used herein, a “low pressure separation” is one that occurs at a pressure below the operating pressure of the GPR so that it becomes necessary to re-pressurize the polymer feed to the GPR. Process units without this equipment may not be capable of producing high MW EPR in the GPR. Moreover, even when an ICP line has the necessary hydrogen separation equipment, GPR transitions to high MW EPR conditions, i.e., low hydrogen concentration in the GPR, can take a relatively long time and generate large quantities of non-spec product for as much as 12 to 24 hours or more. Furthermore, regardless of the hydrogen removal equipment, it can be difficult to control the level of chain terminating agent in the GPR to obtain the desired molecular weight.
There is a need in the art for production systems and methods to transfer more quickly to high MW EPR conditions, and/or in which control of the chain terminating agent concentration and thus the EPR MW is facilitated.